Title: ToO Observations of Cen A in a Bright State


Proposal ID: 0120034
Subject category: Extragalactic Object
Principal investigator: Steinle
Institute: Max-Planck-insitut fuer extraterrestrische Physik

Abstract

We propose to observe Centaurus A (NGC 5128) for 1800 ksec in a high emission state in X- and gamma-rays which has never been observed with modern instruments since the historically high state in the years 1973 - 1976. Cen A is at a distance of less than 4 Mpc and thus the closest radio loud AGN. It provides the opportunity to observe an AGN viewed at a large angle (about 70 degrees) with respect to the jet axis in detail over the whole X-ray and gamma-ray range covered by INTEGRAL. The spectrum is unique among AGN observed at high energies (excluding blazars), with emission detected up to one GeV. In gamma-ray observations by the CGRO instruments from April 1991 to June 2000, Cen A has been observed in intermediate and low intensity states only. Those measurements have shown, that at higher energies (above ~100 keV), the spectral slope and the energy of a further spectral break are changing with the intensity on the observed energy range. These results contradict the long-standing assumption, that the energy output of the source peaks in the MeV region. Cen A appears to be observationally intermediate between Seyfert Type 2 galaxies and blazars. A better understanding of the spectral shape and variability above 100 keV is critical in clarifying this picture. Detailed studies of Cen A in the INTEGRAL energy range (continuum and possible line emission) provide an opportunity to determine whether beaming models or other black-hole accretion scenarios best describe emission from this unique AGN. Observations of Cen A at intensity levels approaching the historically highest observed state would provide greatly improved observation precision and would provide tests of hypotheses concerning intensity-dependent spectral evolution which were advanced to explain the low and intermediate intensity level observations made so far. The RXTE All-Sky-Monitor data, which are online available, will be the primary trigger of theToO.